1. Field of the Invention
This invention relates to a process for crosslinking polymers (including homopolymers and copolymers) and to the crosslinked polymeric products obtained. More particularly, the present invention relates to a process for silane crosslinking wherein the silane is fed into an extruder in a solid form.
2. Description of the Prior Art
The properties of polymers can be modified by crosslinking of the polymer chains. One method for crosslinking which is commonly used in the industry is known as "silane crosslinking", i.e., crosslinking using olefinically unsaturated alkoxysilanes. This method involves at least two stages. In the first stage, a silane is grafted onto base polymer chains in the presence of a free radical generator (e.g. peroxide catalyst) and in a second stage, crosslinking of the polymer chains occurs through the formation of Si-O-Si bonds as bridges. To effect crosslinking, the intermediate formed by grafting is subjected to moisture, for example, hot water or steam, in the presence of a hydrolysis/condensation catalyst. Two processes most often employed are (1) a two-step Sioplas .TM. process as taught in U.S. Pat. No. 3,646,155 and (2) a Monosil.TM. one-step process as taught in U.S. Pat. No. 4,117,195.
A problem associated with both the Sioplas.TM. process and the Monosil.TM. process methods is the difficulty of adequately combining some or all of the silane, peroxide catalyst and hydrolysis/condensation catalyst (some or all of which are liquids) with the base polymer which is a solid. If the silane is injected as a liquid stream into a conventional extruder and mixed with a polymer, small gels form throughout the polymer product, apparently because of localized, premature crosslinking in areas of high additive (silane and catalyst) concentration resulting from inadequate mixing.
When a silane is sprayed into a polymer, additional apparatus is required, and the silane must be sprayed uniformly. High demands are placed on the uniformity of mixing at or near the spraying point to equalize variations in concentration and ensure that the silane is distributed homogeneously.
To prevent localized, premature crosslinking in areas of high additive concentration and to equalize variations in concentration at or near the point where the silane and peroxide are introduced, a long extruder, e.g. such as the one taught in U.S. Pat. No. 4,117,195 may be employed. Furthermore, introducing the silane as a liquid into the conveying section of an extruder typically results in slippage and poor metering. The use of a long extruder also improves metering in such cases.
U.S. Pat. No. 4,247,498 describes microporous products and a method for making microporous products which can absorb relatively large amounts of "functionally useful liquids" and still behave as solids. The microporous products are made from synthetic thermoplastic polymers and "compatible liquids." In the course of producing the products, some of the compatible liquids are removed. The compatible liquids may also be functionally useful liquids and, when they are, the liquid remaining in the product permits the product to be used directly. A variety of such dual purpose liquids are disclosed at Column 27, line 62 to Column 28, line 32. Table VIII in this patent illustrates the formation of homogeneous porous polymer intermediates from polypropylene and compatible liquids. Footnote (1) suggests that phenylmethyl polysiloxane was used as a compatible liquid. However, this patent does not mention using silane crosslinkers.
U.S. Pat. No. 4,783,511 describes a process for the preparation of molded bodies from dry thermoplastic polyamides which have improved mechanical properties and higher shape-stability than the polyamide starting materials. The process involves adding to the dry polyamide a masterbatch containing a silane and a "substrate" (i.e., a polyolefin, a swellable material or a non-swellable thermoplast sponge) before processing the dry polyamide. Three methods of masterbatch preparation described in the patent (Column 3, lines 11 ff.)are summarized as follows:
(1) A melt of a polyolefin is mixed with the silane and the silane-containing melt is drawn off as a cord which is comminuted to give a granulate which is carefully dried. PA1 (2) A silane-swellable particulate carrier material (for example, in the form of crumbs, grains or a coarse powder) is contacted with the silane, so that the silane is directly absorbed on the carrier. The carrier must remain sprayable, i.e., there should be no adhesion of the powder particulate. PA1 (3) A thermoplast, which is in the form of a sponge with open pores but which has no swellability with respect to the silane, is mixed with the maximum amount of silane sufficient to fill the pores. The sprayability of the thermoplast should be essentially unchanged.